Method of producing water-insoluble mineral salts



7 Claims. (Cl. 167-85) The present invention relates to water-insoluble compound basic mineral salts of the type consisting of a mixture in predetermined proportions of salts of at least one metal and at least two acids, of which at least one is carbon dioxide, and containing also at least one oxidation compound of this metal.

The invention concerns more especially bismuth subnitrates and their process of production.

An object of the invention is to provide a process for producing salts of the aforesaid type which have a relatively low apparent density, and therefore ofiering certain interesting inherent properties: such as that of being endowed with a much greater covering power than when they are prepared in denser compact form.

Another object of the invention is to produce such salts having a metal content higher than that of analogous salts prepared by methods leading to the production of basic salts of higher apparent density.

A more specific object of the invention is to provide a method of manufacturing a new extra-light bismuth subnitrate.

The bismuth salts, and more especially the subnitrates, are well-known compounds in their conventional form and afford a convenient comparison between the conventional compounds and processes and those according to the invention. Accordingly, the invention will hereinafter be described with reference to bismuth subnitrate by way of example, with the object of facilitating the understanding of the invention, but this does not in any way constitute a limitation of the invention.

Thus, for example, the French Pharmacopoeia (Codex Francais, 1-949) distinguishes between three types of bismuth nitrates which are characterised by their bismuth content calculated as bismuth oxide Bi O These are: neutral bismuth nitrate (48% of Bi O heavy basic bismuth nitrate or heavy bismuth subnitrate (7981% of Bi O and light basic bismuth nitrate or light bismuth subnitrate (minimum of 82% and maximum of 89% of Bl203)- It is a particular object of the invention to produce a new variety of extra-light bismuth subnitrate, of which the bismuth content, expressed as Bi O exceeds 90%. This content is thus clear-1y higher than that of the conventional basic bismuth nitrates. The apparent density of this new bismuth subnitrate is lower than 0.3, or at least twice as low as that of the Codex hea'vy subnitrate (0.76) and of the Codex light subnitrate (0.6).

The conventional process for the production of light basic bismuth nitrate or light subnitrate as described in the French Pharmacopoeia consists in precipitating a bismuth nitrate solution in an alkaline medium. The salt obtained consists of a mixture in variable proportions of bismuth hydroxide, basic bismuth carbonate and basic bismuth nitrate. It, on the other hand, also in accordance with the French Pharmacopoeia, bismuth nitrate is hydrolysed in nitric acid medium, heavy basic nitrate is obtained.

In contrast thereto, a further object of the invention is to produce an extra-light bismuth subnitrate by hydrolysing an acid solution of neutral bismuth nitrate in an ammoniacal medium, in the presence of compounds capable of liberating free carbon dioxide in the course of the precipitation, such for example as ammonium carassists Patented lune 42-, 1963 bonate, the respective concentrations of the various reactants and the operating conditions determining the composition of the compound basic salt thus obtained, as also its apparent density.

Yet another object of the invention is to provide a process of manufacturing salts of the aforesaid type, comprising the step of dissolving the metal or metals in the acid or acids other than carbon dioxide, so as to obtain an aqueous acid solution in a predetermined state of dilution of salts of the said metal or metals, of which the free acid content is just sufficient to prevent hydrolysis of the said salts.

Still another object of the invention is to provide such a process, further comprising the step of rapidly pouring the said acid solution into an alkaline solution in a predetermined state of dilution, in the presence of a predetermined quantity of labile CO ions so as to precipitate the said compound basic salts by hydrolysis.

The invention will be more readily understood from the following detailed description of a non-limiting example of the application of the invention.

To prepare an extra-light bismuth subnitrate according to the invention, a bismuthic liquor is first prepared. For this purpose, there are first weighed about 200 kg. of metallic bismuth containing at least 98.5% of Bi free from arsenic, but which may contain traces of lead, copper or silver, as also chlorides and sulphates. The bismuth is melted and brought to a temperature above its melting point, that is to say, about 300 C. The impurities floating on the surface of the molten metal are decanted and the molten metal is slowly and carefully poured into a vat containing about 200 litres of demineralised water. The relatively high temperature to which the metal is brought before being granulated permits of obtaining finer grains which are easy to handle. The granulated bismuth thus obtained is introduced into a 1000-litre stainless steel tank. There are poured on to this granulated metal, at ambient temperature, about 170 litres of demineralised water and, gradually, over a period of about 1 hour, about 420 litres of commercial concentrated nitric acid (density about 1.32). The very exothermic reaction commences immediately and the temperature of the mixture may reach about 70 C. An intensive liberation of nitrous vapours occurs, these vapours being discharged into the atmosphere after having been neutralised by a conventional method, for example by passing them through a plastic tank in which ammonia in fine droplets is sprayed on to beds of carbon.

This method of operation is particularly practical, but it is also possible to prepare a dilute acid solution by mixing similar quantities of acid and water before pouring this solution onto the metal or again the metal can be introduced into the acid before or after dilution.

When the reaction is complete, and after cooling to ambient temperature, a yellowish solution is obtained which ultimately becomes decolorised, and the decoloration of which can be activated by oxidation should it be desired subsequently to obtain a salt not having a yellowish tint. In order to decolorise this solution, there is added thereto about 1 kg. of urea previously dissolved in about 10 litres of water, and the mixture is homogenised by bubbling air therethrough. This decolora-tion may also be effected by another method of oxidation, for example by bubblinghrough oxygen or even simply leaving the solution in contact with the air for a lengthy period. With the urea treatment, there are thus obtained aiiter standing for 24 hours about 600 litres of a colorless liquor having a density equal to about 1.56 and having a pH value of about 1. The bismuth is then entirely brought into solution in nitric acid in the form of neutral bismuth nitrate to the threshold of hydrolysis.

It is to be noted that, the more dilute the solution, the lower is the density of the compound basic salt subsequently obtained. However, a limitation of the dilution is necessary for the convenience of handling. The aboveindicated proportions constitute a particularly advantageous form of application of the invention, but they have no limitating character.

The hydrolysis is carried out in a stainless steel tank having a capacity of about 1000 litres, into which there are successively introduced about 650 litres of demineralised water, about 52 litres of ammonia (of a density of about 0.940) and about 6 kg. of ammonium carbonate previously dissolved in about 10 litres of demineralised water. After mixing, there is thus obtained an alkaline bath whose density is about 0.997, whose pH value is about 9.6 and whose NH titre is about 1.25%. The alkaline bath is heated by means of a steam coil or any other appropriate heating means to about 41 C. There are then very rapidly poured into this alkaline bath 100 litres of the nitric acid solution of neutral bismuth nitrate having a pH value of about 1 and a density of about 1.56, prepared as above. The period of introduction is advantageously less than about 1 /2 minutes for the quantities indicated. Mixing is thereafter mechanically elfected with a stirrer for not more than about 4% minutes and it is verified at the end of the reaction that the pH value is still alkaline (about pH=8.4) by the bromothymol blue reaction. If the solution is slightly acid or even neutral at the end of the operation it may still contain some bismuth salts in solution. These salts can be recovered by the very rapid addition of a suflicient quantity of ammonia, the stirring being continued. However, the compound basic salt obtained in this case is less light than that obtained with correct proportions of the constituents such that the bath is still alkaline after total neutralisation of the nitric acid solution. The rapidity of introduction of the nitric acid solution and the limitation of the stirring period to some extent determine the lightness of the compound basic salts thereafter obtained. This is probably due to the liberation of CO which, in the case of rapid addition, takes place simultaneously with the formation of the precipitate, while in the case of slow addition the CO may be liberated from the solution at the very beginning of the addition of the acid slution and may be completely liberated before the end of the hydrolysis.

This hydrolysis involves the precipitation of the desired compound basic salts in the form of a white precipitate of clotted appeanance, which is decanted and introduced into a centrifuge, the bowl of which has a diameter of about 75 cm. and rotates at about 1200 rpm. The precipitate is washed in the centrifuge for about 50 minutes with about 200 litres of demineralised water at ambient temperature. The speed of the centrifuge is then raised to about 1500 r.p.m. for approximately 40 minutes. There is thus obtained a cake containing about 70% of water and 0.02% of ammonia. This cake is thereafter introduced in small fractions into the top of drying tower comprising, for example, 8 plates having a diameter of about 1 metre which are stacked over a height of about 3 m., in which tower the cake is driven by ro tative blades sweeping the plates. The said blades cause the product to descend successively from plate to plate in the opposite direction to an ascending current of hot air introduced through the lower end of the tower, at a temperature of about 90 C. The cake is gradually converted into a white powder, the lighter particles of which are carried along by the hot air current and recovered at the outlet of the tower in a cyclone-type decompression chamber maintained at about 70 C. The drying period in the tower is advantageously of the order of l2-24 hours. On leaving the tower, the product takes the form of a white powder containing less than 1% of humidity, which can be finely crushed, if desired, for example in a hammer mill comprising a stainless steel grid having meshes measuring about mm., such as that known under the trade name Forplex No. 0.

The yield of the operation is excellent (about 99% of the theoretical yield calculated on the initial metallic bismuth), so that no recovery of the mother liquors of the precipitation is necessary. The bismuth is completely precipitated if care is taken to use an alkaline hydrolysis bath containing a slight excess of ammonia over the acid contained in the hydrolysed nitric acid solution.

The centrifuging and drying conditions also determine to some extent the apparent density of the final product. More prolonged centrifuging, which leads to a degree of humidity less than 70%, gives products whose apparent density after drying is higher. This humidity of about 70% also corresponds to the industrially acceptable cen trifuging limit. In fact, more intensive elimination of this water of retention involves the use of a greater amount of centrifuging equipment and takes a relatively long time. In addition, it is found that it is necessary to ensure that the precipitated product does not undergo any untimely shock or compression until it has reached a very high state of dryness, as otherwise the apparent den sity of the final product is higher, as is the case with more vigorous centrifuging.

The compound basic salt thus obtained is white, while it has a yellowish hue if the nitric acid solution has not been decolorised. It has an apparent density less than 0.3, which may even fall to about 0.19, and a. bismuth titre, expressed as B1 0 of the order of -92%. It consists of a mixture of hydroxide, carbonate and basic nitrate of bismuth, the composition of which is substantially the same in each prepared batch under the same operating conditions. It is free from silver, copper, lead and alkaline-earth salts and obviously also free from arsenic. It may contain very slight, substantially indeterminable traces of sulphates, chlorides and alkali salts. This extra-light bismuth subnitrate has a more argeeable flavour than the conventional subnitrate of the type contained in the French Pharmacopoeia, probably due to the absence of alkali metal ions or alkalineearth-metal ions.

The following table shows in percentages the comparative analyses of the extra-light subnitrate obtained and those of the light and heavy subnitrates given in the French Pharmacopoeia.

Extra-light, percent 4to8 8to14 15.5to17 7 to 8 1 to 2 0 90 to 92 82 to 89 79 to 81 This new subnitrate is particularly advantageous in most applications of the conventional light subnitrate" of the type referred to in the French Pharmacopoeia. Its pharmaco-dynamic activity is more especially higher than that of the heavy and light subnitrates in the French Pharmacopoeia, from which it differs by lower toxicity by reason of its lower content of N 0 ions, and by higher activity, by reason of its higher Bi titre and its lower apparent density. This combination of properties places it in a different category from the known bismuth subnitrates. It is also more advantageous both in cosmetic and in ceramic applications, for example for the production of iridescent effects or for gilding earthenware, as also as a flux in vitrified paint applied to glass and porcelain. Its low apparent density imparts thereto a covering power very much greater than that of conventional products, whereby its use is rendered distinctly more advantageous.

It will be obvious to chemists that the invention is not limited to the compounds and to the method of preparation which have been mentioned. Thus, instead of ammonium carbonate, the use of which is particularly advantageous by reason of its ready application, its low cost and the excellent results which it gives, there may be employed any solid, liquid or gaseous compound which is capable of producing during the hy drolysing reaction a regular and homogeneous evolution of carbon dioxide. For example, carbon dioxide itself, carboxylic acid, their derivatives or their salts, as also any suitable chemical compound may be employed, depending upon the object aimed at. If ammonia is replaced by another base, there are also obtained by the process of the invention compound salts which are lighter than those obtained by the conventional processes, but which have higher apparent density than those obtained with ammonia.

What is claimed is:

l. A method for producing a water-insoluble compound basio bismuth nitrate containing 4% to 8% N from 7% to 8% CO and from 90% to 92% bismuth, and having a density lower than 0.3, comprising the steps of granulating 200 parts by weight of fused metallic bisninth in 200 parts by weight of demineralized water; dissolving the granulated metallic bismuth in 200 parts by weight of water and 555 parts by weight of nitric acid having a density of about 1.32; decolorizing the resulting acid solution of bismuth nitrate by adding thereto about 1 part by weight of urea dissolved in 10 parts by weight of demineralized Water; rapidly pouring, while stirring 100 parts of said acid solution having a density of about 1.56 and a pH value of about 1 into an alkaline bath at about 41 C., said alkaline bath having a density of about 0.997, a pH value of about 9.6, an NH; titre of about 1.25%, and being constituted by about 660 parts by weight of demineralized water, about 49 parts of ammonia having a density of about 0.940, and 6 parts of ammonium carbonate, to thereby precipitate said com pound basic bismuth nitrate; recovering the precipitated compound bismuth nitrate; centrifuging the latter under controlled conditions to reduce its H O content to a value of the order of 7%, drying said compound basic bismuth nitrate in a current of hot air having a temperature of 70 C. to 90 C., and thereafter crushing s-aid compound bismuth nitrate until the latter is in the form of a fine powder.

2. A method for producing a water-insoluble compound basic bismuth nitrate containing hydroxide, carbonate and basic nitrate of bismuth in selected proportions, comprising the steps of providing an aqueous acid solution of bismuth nitrate having a pH of about 1; rapidly pouring said acid solution into a dilute alkaline solution of ammonia containing carbonate ions and having a pH of about 9.6, to precipitate a water-insoluble compound basic bismuth nitrate; washing the precipitated compound basic bismuth nitrate with water, and drying the same under controlled conditions, while preventing said compound basic bismuth nitrate from undergoing excessive shock and compression loads.

3. A method for producing a water-insoluble compound basic bismuth nitrate containing hydroxide, carbonate and basic nitrate of bismuth in selected proportions, comprising the steps of providing an aqueous acid solution of bismuth nitrate having a pH of about 1; rapidly pouring said acid solution into a dilute alkaline solution of ammonia and ammonium carbonate having a pH of about 9.6 to precipitate said water insoluble compound basic bismuth nitrate; washing the precipitated compound basic bismuth nitrate with water and drying the same under controlled conditions, while preventing said compound bismuth nitrate from undergoing excessive shock and compression loads.

4. The method according to claim 2, wherein the steps of drying said compound basic bismuth nitrate is car ried out by means of a stream of hot air having a temperature of substantially C.

5. The method according to claim 3, further comprising the steps of crushing said compound basic bismuth nitrate after drying the same, until a powder of desired fineness is obtained.

6. The method according to claim 3, wherein the alkalinity of said dilute solution of ammonia and ammonium carbonate has a value higher than the neutralizing equivalent of said acid solution.

7. A method for producing a water-insoluble compound basic bismuth nitrate containing hydroxide, carbonate and basic nitrate of bismuth in selected proportions, comprising the steps of providing an aqueous acid solution of bismuth nitnate having a pH of about 1; purifying said acid solution by introducing into the same an oxidation agent; precipitating said water-insoluble compound basic bismuth nitrate by rapidly pouring said acid solution into a dilute alkaline solution of ammonia and ammonium carbonate having a pH of about 9.6; washing the precipitated compound basic bismuth nitrate with water and drying the same by means of a stream of hot air having a temperature of substantially 90 C., while preventing said compound basic bismuth nitrate from undergoing excessive shock and compression loads.

References Cited in the file of this patent UNITED STATES PATENTS oretical Chemistry, Longmans, Green and Co., vol. 9, 1929, pages 706-7.

Hackhs Chemical Dictionary, 3rd Ed, 1944, page 813. Thorpe: Thorpes Dictionary of Applied Chemistry, Longmans, Green and Co., 4th Edition, 1937, vol. 1, pages 699-700. 

2. A METHOD FOR PRODUCING A WATER-INSOLUBLE COMPOUND BASIC BISMUTH NITRATE CONTAINING HYDROXIDE, CARBONATE AND BASIC NITRATE OF BISMUTH IN SELECTED PROPORTIONS, COMPRISING THE STEPS OF PROVIDING AN AQUEOUS ACID SOLUTION OF BISMUTH NITRATE HAVING A PH OF ABOUT 1; RAPIDLY POURING SAID ACID SOLUTION INTO A DILUTE ALKALINE SOLUTION OF AMMONIA CONTAINING CARBONATE IONS AND HAVING, A PH OF ABOUT 9.6, TO PRECIPITATE A WATER-INSOLUBLE COMPOUND BASIC BISMUTH NITRATE; WASHING THE PRECIPITATED COMPOUND BASIS BISMUTH NITRATE WITH WATER, AND DRYING THE SAME UNDER CONTROLLED CONDITIONS, WHILE PREVENTING SAID COMPOUND BASIC BISMUTH NITRATE FROM UNDERGOING EXCESSIVE SHOCK AND COMPRESSION LOADS. 